1. Field of the Invention
The present invention relates to an oscilloscope intensity regulation apparatus.
The invention is particularly concerned with an apparatus of an oscilloscope for displaying wave-forms with a regulated intensity in spite of variations of repetition rates of the wave-forms and sweep velocities in the oscilloscope.
2. Description of the Prior Art
In an oscilloscope with a cathode-ray tube (CRT) for displaying wave-forms, low repetition rate wave-forms swept at high speed are displayed with insufficient intensity. It is, therefore, difficult to observe the wave-forms displayed with insufficient intensity on the CRT.
There is an oscilloscope with a charge coupled device (CCD) sensor on a surface of the CRT. The CCD sensor obtains a video signal for displaying the wave-forms. Wave-forms with enough strong intensity are observable by means of video signal via the CCD sensor without direct viewing on the CRT.
Shown in FIG. 1 is a circuit block diagram of a prior art oscilloscope with a CCD sensor. A CRT 10 displays wave-forms to be observed on its surface. A Z-axis circuit 8 is connected with a cathode of the CRT 10. The Z-axis circuit 8 controls a beam current of the CRT 10 in accordance with instructions from a central processing unit (CPU) 4. An observer controls an intensity input 2 to obtain a desirable intensity of displayed wave-forms. The intensity input 2 is, for example, a variable register or a potentiometer delivering an intensity input signal 21 to the CPU 4. The CPU 4 instructs the intensity to the Z-axis circuit 8 according to the signal 21.
The CCD sensor 11 is on the surface of the CRT 10. The CCD sensor 11 reads wave-forms on the surface of the CRT 10 to deliver a CCD output 28 to a video circuit 12. The circuit 12 provides an analog to digital (A/D) converter 14 with a video signal 29. A display 19 displays the video signal converted to digital.
In FIG. 2, there are shown the CRT 10, the CCD sensor 11 and an output thereof. The output of the Z-axis circuit 8 is supplied between a cathode K and a grid G of the CRT 10. The cathode K emits an electron current beam in accordance with the output of the Z-axis circuit 8. The beam is deflected by deflection plates to display bright wave-forms on a fluorescent screen 9 of the CRT 10. The CCD sensor 11 converts the bright wave-forms to an electrical signal.
A curve S shows a variation of an intensity I depending on X-position, in which the center of a bright trace is X.sub.0. The maximum intensity is shown at the center position X.sub.0 of a width of the bright trace in view of an arrow 51.
In FIG. 3, there are shown curves S.sub.a, S.sub.b, S.sub.c of (a) and curves D.sub.a, D.sub.b of (b).
In FIG. 3(a), the curve S.sub.a is the same as the curve S of FIG. 2. At the center position X.sub.0, the curves S.sub.a, S.sub.b, and S.sub.c show respectively the intensities I.sub.a, I.sub.b and I.sub.c, in which I.sub.a &lt;I.sub.b &lt;I.sub.c. The width of the bright trace of the curve S.sub.b is wider than that of the curve S.sub.a, In the same manner, the width of S.sub.c is wider than that of S.sub.b. The bright trace of S.sub.c shows a halation.
In FIG. 3(b), there are curves D.sub.a and D.sub.b. Curves D.sub.a and D.sub.b show variations of intensities depending on outputs of the Z-axis circuit 8. The curve D.sub.a is in case of low repetition rate of a wave-form to be observed and D.sub.b is in case of high repetition rate.
When an output Z.sub.a of the Z-axis circuit 8 is applied to the cathode K of the CRT 10, a wave-form of a low repetition rate is displayed on the fluorescent screen 9 with an intensity I.sub.a on the curve D.sub.a. When output Z.sub.b or Z.sub.c of the Z-axis circuit 8 is applied to the cathode K of the CRT 10, a wave-form of a high repetition rate is displayed on the fluorescent screen 9 with an intensity I.sub.b or I.sub.c on the curve D.sub.b. In spite of the fact that the outputs Z.sub.a, Z.sub.b and Z.sub.c are close in magnitude and show a relation of Z.sub.b &lt;Z.sub.a &lt;Z.sub.c, intensities I.sub.a, I.sub.b and I.sub.c show a relation of I.sub.a &lt;I.sub.b &lt;I.sub.c. Differences among intensities I.sub.a, I.sub.b and I.sub.c are quite big. It means that intensity differences depend considerably on repetition rates of wave-forms being displayed.
It is difficult to observe the low repetition rate wave-form displayed on the fluorescent screen 9, because the intensity I.sub.a is not enough for observing with the naked eye. For resolving the problem, there is a method employing a high acceleration voltage of an electron beam in the CRT 10 to strongly excite the fluorescent screen 9. In the method, there is problem that it requires a high acceleration voltage accompanying with the deterioration of the deflection sensitivity and the fluorescent screen 9 burnt up.
For avoiding those problems, the CCD sensor 11 of FIGS. 1 and 2 is employed so as to obtain bright traces of wave-forms. As shown in FIG. 3, intensities significantly depend on repetition rates of wave-forms being displayed. Trace widths of displayed wave-forms vary in wide range with halations sometimes.